STRUCTURAL CONTROLS ON PALEOPROTEROZOIC IRON FORMATION-HOSTED GOLD MINERALIZATION IN THE HOMESTAKE MINE AREA, LEAD, SOUTH DAKOTA USA

The Homestake deposit, host for > 40 million ounces of gold, is located on the west limb of a SE-plunging regional anticlinorium (F₂) formed during regional deformation (D₂) at greenschist to amphibolite facies during the Black Hills orogeny (~1750 Ma). Ore mineralization was localized in the Lead anticline and syncline pair that were formed from complex progressive deformation associated with sinistral transpression at ~1740 Ma. Structural analysis and application of models of transpression provide new insights into the distribution of gold mineralization.

Lower strains observed in competent amphibolite unit in the core of the major anticlinorium allowed two deformation events (D_2a and D_2b) to be preserved. Structural observations and measurements indicate that the cleavage (S_2a) is highly deformed and concordant with compositional layering. D_2a folds (F_2a) plunge moderately to the SE or ESE. D^_2b is associated the dominant steeply east dipping cleavage (S_2b) that is axial planar to folds (F_2b) that trend SSE and plunge gently to steeply depending upon the type, degree and orientation of later shearing.

Analysis within iron formation and weaker phyllite indicate that earlier structural elements related to D2a and D2b were localized into four different upright planar domains. Two relate to the earliest formed West Ledge shear system that parallel the regional deformation fabric (N15W) and the later formed, but contemporaneous East Ledge shear system that strikes N35W that formed as a result of localization of strain along the margin of the amphibolite unit. The third (N50W) is interpreted to contain the oblique flow apophysis for a monoclinic deformation. This indicates shear systems are pure shear-dominated (west) and wrench dominated transpression (east). This agrees with steeply plunging lineations in the West Ledge shear system and gently plunging lineations in the East Ledge shear systems. The fourth domain strikes N-S and may be related to shear zones localized along the N-S striking limb of the major anticlinorium or possibly a slightly younger event.

The intersection of the East and West ledge shear systems is coincident with the the deposit and explains for the geometry of the Lead anticline and syncline pair, the orientation of ore ledges and a partial explanation for the orientation of ore shoots.